Field of the Invention
The invention relates to a dentists' preparation instrument having a turbine for driving a tool by means of compressed air. A rotor disposed in a turbine compartment and rotating about a longitudinal axis has blades extending to a face of the rotor. The turbine compartment has a discharge port for the compressed air to a return air duct, wherein the discharge port is disposed in the turbine compartment in such a way that the face of the rotor passes by the discharge port laterally in rotation about the longitudinal axis.
Description of the Related Art
A dental turbine handpiece is known from EP 0 974 308 B1 with which, after the turbine has been shut down, because of the reduced pressure that develops due to after running in a turbine compartment upstream from the discharge port, a curtain flow is created for the purpose of preventing intake or reverse draw. This takes place due to the fact that a flow web protruding into the flow path deflects the flow from the discharge port inward. A flow groove may be placed upstream from the flow web disposed in the turbine compartment to make the flow step greater and to increase the efficiency. The compressed air flows out on one face of the blades and into an annular chamber provided in the turbine wheel, where it loses pressure. Both the annular chamber and parts of the blades lie opposite the discharge port. During functional operation, however, there is a break in the curtain flow because of the pressure gradient.
DE 100 60 152 B4 describes a dental turbine handpiece in which a rotor is equipped with a first and a second turbine wheel to increase the torque, and connecting ducts are provided to deflect the pressure medium from the first turbine wheel to the second turbine wheel. In one embodiment, the flow passes twice through the second turbine wheel, which decelerates the rotor and reduces the rotational speed.
The object of the invention is to achieve a reduction in the idling speed in order to improve the lifetime and noise behavior without restricting the effective power to an extent that would be of practical relevance.